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AN1620

APPLICATION NOTE

November 2002

INTRODUCTION

This note describes an off-line autoranging (85Vac-264Vac) battery charger designed to operate in three differ-

ent functional modes:

– constant voltage

– constant current

– constant power

These requirements are typical of battery chargers for notebook computers.

The resulting output characteristic of the system is shown in fig 1.

The A-B portion of the V-I characteristic is a constant voltage mode. This situation happen when the battery

pack is almost completely charged. Vo has to be set at the final value of the battery pack. From point B to C the

charger operates in constant power mode, reducing the output voltage while increasing the output current.

The benefit of introducing the constant power mode is that the maximum output power is lower with respect to

the power output at point E. This reduces the power managed by the primary circuitry and in consequence re-

duces the total dissipation power. This is a useful improvement considering that the complete system is pack-

aged in a plastic case with a very limited capability of dissipating heat.

From point C to D the adapter works in constant current mode until the output voltage reaches 0V.

In practice it is difficult to have a constant current characteristic below an output voltage of 2-3V because the

primary auxiliary supply voltage drops at the controller turn-off threshold (internally fixed at 7.6V typ.) and the

V-I characteristic exhibit a foldback to zero.

If mandatory to operate in constant current mode till 0V, a secondary post-regular or other tricky solutions on

the windings, primary or secondary, are required.

Fig 2 shows the V-I characteristic obtained with the circuit of Fig 3.

ELECTRICAL SPECIFICATION:

Input voltage

85Vac to 264Vac

Regulated output voltage

18V

Maximum output power

25W

Switching frequency

100kHz

Target efficiency @ full load

>80%

Topology

discontinuous current mode flyback

Fig 3 shows the electrical circuit.

Figure 1. V-I output characteristic

Figure 2. Practical V-I characteristics

AB

E

C

DI

V

D96IN437

0

0.5

1.0

1.5

2.0

0

5

10

15

D96IN439

by Domenico Arrigo

25W OFF-LINE AUTORANGING BATTERY CHARGER

WITH L5991A